Patent GB-1323973-A (GB)
Title: Magnetic imaging
IPC: G03G19/00
Filed: 1971-04-19
Abstract: 1323973 Photoconductive device BELL & HOWELL CO 19 April 1971 [17 April 1970] 26831/71 Heading H1K [Also in Divisions B6, G2 and H3] A recording medium comprises interdigitated electrodes, a photoconductive layer in electrical contact with the electrodes and a magnetic recording layer in heat-transfer relationship with the photoconductive layer. In an embodiment a transparent sheet 13 of glass, polyimide or polybenzamidazole is provided with interdigitated electrodes 15, 16 of Au, Al, In or Cr. A photoconductive layer 18 comprising CdS, CdSe and ZnS is deposited on the substrate and electrodes. A magnetic recording layer 20 having a low Curie temperature is either joined to or pressed against the photoconductive layer 18. The recording layer 20 may exhibit an acute transition between the ferromagnetic and the paramagnetic state or comprise magnetic particles with a shape anisotropy that dominates their crystal anisotropy. In one method of using the medium, it is uniformly magnetized and selectively demagnetized in an image-wise pattern by heating to above the Curie temperature of the layer 20. Alternatively selected portions of the unmagnetized layer 20 are heated to above the Curie temperature in an image-wise pattern and allowed to cool in the presence of a magnetic field. A combination of these techniques may be used. In an embodiment using the first technique to produce a premagnetized layer 20, a sheet of light 26 emerging from shade 24 is moved across a reflector or sheet of paper 53 by the rotation of mirror 28 thus scanning the photoconductive layer 18 with a band of light 50. The band 50 renders the layer 18 sufficiently conductive to allow a heating current to pass and thereby raise the layer 20 above its Curie temperature, whilst a magnetizing head 40 moves synchronously but slightly behind the band 50. If a D.C. source 44 is used to energise the head 40 then a series of magnetized lines parallel to the electrodes 15, 16 are obtained in between the electrodes where the photoconconductive layer 18 has been exposed. Alternatively if a point-pattern rather than a linepattern is required then the sheet 53 is provided with a pattern of reflecting and non-reflecting portions or an A.C. source 44 is used. The medium is exposed by scanning an imagebearing master 31 with the sheet of light but without simultaneously using the magnetizing head 40. The electrodes are biased so that sufficient current flows through the exposed portions of the photoconductive layer to raise its temperature and thereby heat layer 20 above its Curie temperature. The magnetic image (68) thus formed can be stored and printed out as often as desired. Development (Fig. 3, not shown) is effected by applying magnetic toner (70) to the image (68) and transferring the developed image to a sheet (75) of paper or film optionally bearing an adhesive layer (76). The toner may be powdered Fe, Fe 3 O 4 , Co or Ni which may be used as a liquid suspension, a magnetic ink or particles having a fusible shell. Erasure of the image is effected by subjecting layer 20 to anhysteretic magnetic erasing fields or by simultaneously uniformly flooding the photoconductive layer 18 with light and passing a current through the electrodes to raise the magnetizable layer above its Curie temperature.